The present invention relates to a photometric circuit incorporated in an objective lens assembly including an automatic diaphragm for a closed circuit television (CCTV) camera.
In an objective lens assembly provided with an automatic diaphragm so arranged that the DC voltage obtained by rectifying video signals coming from the television camera are utilized as the control signals for diaphragm control to obtain the optimum image, the photometric circuit serving as means for detection of an object to be picked up is very important, since the photometric or light measuring mode for this objective lens depends on the capabilities of such a photometric circuit.
To obtain the optimum image, not only absolute brightness of an object to be picked up, but also light and shade ratios of different areas in the same object, i.e., contrast, are important factors to be considered. The contrast lies within a range of tens: one even in cloudy daylight and within a range of hundreds: one in cloudless daylight. In the image reproduced by a television, on the contrary, the contrast lies within a range as narrow as of 30 to 40:1. When a bright area and a dark area are simultaneously contained in the same object, therefore, it is necessary to decide whether said bright area is selected as the main object or said dark area is selected as the main object to be picked up, since it is impossible to see both areas at the same time.
In principle, a so-called average light measuring mode is suitable for the case in which the dark area is selected as the main object to be picked up, while a so-called peak light measuring mode is suitable for the case in which the bright area is selected as the main object to be picked up. These light measuring modes for a video system usually employ the video signals coming from the television camera to obtain a diaphragm control signal. However, rigid use of these light measuring modes would cause inconveniences. Specifically, in the case where the same object simultaneously contains the bright area and the dark area as previously mentioned, the average light measuring mode would often make the bright area a white solid area without any gradation while the peak light measuring mode would often make the dark area, which should be adequately discriminated, indiscriminate. The average average light measuring mode and the peak light measuring mode should be selectively adopted depending on whether the dark area or the bright area is used as the main object to be picked up when these two areas simultaneously exist in one and the same object to be picked up. Accordingly, it is preferred that a video signal of the same level is obtained regardless of the modes so far as the object contains no dark area, i.e., no contrast state is concerned.
In view of the fact that a condition of an object to be picked up is sensitive to not only said light and shade ratio, i.e., the contrast of the dark area and the bright area, but also the proportions of whole the object occupied by these areas, respectively, the present invention provides a photometric circuit permitting the light measuring modes to be selectively converted and, at the same time, permitting a video signal of the same level to be obtained even when the light measuring mode conversion takes place so far as the no contrast state is concerned.
FIG. 2(a) shows a test pattern in which a central blank area represents a bright area and a shaded area surrounding said blank area represents a dark area. In this test pattern, a proportion of the whole object occupied by the bright area, i.e., the ratio of bright area S.sub.o is expressed by EQU S.sub.o =(a/b).sup.2 .times.100 (%).
S.sub.o =100 (%) is established when the whole object is covered by the bright area and contains no dark area, i.e., in the no contrast state.